Easy Thermo Sudoku

Digits must increase from the bulb end to the tip along each thermometer. Temperature direction constrains every placement.

Easy Medium Hard Expert

New to Thermo Sudoku? Digits must strictly increase from the bulb to the tip of each thermometer. Start with the longest thermos for quick wins.

5
6
1
3
4
7
2
9
3
9
7
5
9
8
3
4
4
5
9
8
5
4
3
1
2
9
5
8
7
1
6
4
5
2
5
8
7
3
Mistakes
0/3
Score
-
Time
00:00
Try Medium Thermo Sudoku →
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What is Thermo Sudoku?

Difficulty
★★★★☆
4/5
Constraint Type
Line Constraints
Typical Givens
20–28
Avg. Solve (Easy)
10 min

Solving Techniques for Easy Level

Technique Description Level
Range Restriction from Length A thermometer with N cells must use N strictly increasing distinct values. The bulb is at least 1 and the tip is at most 9, giving a calculable valid range for each cell. Beginner
Minimum/Maximum Bounding The bulb value is ≤ 9 − (N − 1). The tip value is ≥ N. Apply these bounds immediately to prune all thermometer candidates. Beginner

Ready to go deeper? Try Medium Thermo Sudoku to unlock Intermediate techniques.

Techniques to Master at Easy

  • Last Possible Number technique — Apply length bounds first — a bulb can be at most 9−(length−1) and a tip at least the thermometer's length — and many thermometer cells are left holding one legal digit.
  • Notes in Sudoku — Pencil-mark each thermometer cell with its min–max window before anything else; every placement on the line ratchets the windows of all cells bulbward and tipward.

Average Solve Time by Difficulty

Easy
10 min
Medium
22 min
Hard
45 min
Expert
75 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Thermo Sudoku →

Related Variants

Arrow Sudoku

Thermo and Arrow both constrain digits along drawn lines — increase swaps for summation, but the cell-by-cell bounding feels the same.

German Whispers

Another drawn-line rule: instead of strictly increasing, neighbouring line digits must differ by at least 5.

Renban Sudoku

Line cells must form a consecutive set rather than an increasing one — the same range-tracking instinct, loosened by one degree.

Frequently Asked Questions — Easy Thermo Sudoku

What is Thermo Sudoku?
Thermo Sudoku is a Sudoku variant that adds thermometer-shaped clues overlaid on the grid. Each thermometer has a bulb (circular end) and a tip. The rule is simple: digits must strictly increase from the bulb to the tip. Standard Sudoku rules still apply — every row, column, and 3x3 box must contain 1 through 9 exactly once.
How do I read the thermometer clues?
Each thermometer is drawn as a grey shape on the board. The round circle is the bulb — the starting end. The line extends to the tip. A digit placed in any cell on the thermometer must be greater than the digit in the previous cell (closer to the bulb) and less than the digit in the next cell (closer to the tip). Strict increase means no ties are allowed.
What digits can go at the start of a thermometer bulb?
The bulb must hold the smallest digit on its thermometer. On a 3-cell thermometer, the bulb can be at most 7 (since the cells after it need at least 8 and 9). On a 5-cell thermometer, the bulb can be at most 5. Use thermometer length to set hard upper and lower bounds on each position.
How is Thermo Sudoku different from regular Sudoku?
Regular Sudoku gives you pre-filled digits. Thermo Sudoku typically gives you few or no pre-filled digits — instead the thermometer constraints do the work of limiting where each digit can go. The inequality relationships between cells replace the explicit given numbers as your primary source of information.
What is the best first move in easy Thermo Sudoku?
Find the longest thermometers. A 7-cell thermometer has a bulb that must be 1 or 2 and a tip that must be 8 or 9 — these extreme-position deductions are the easiest wins. Also look for thermos where two positions share a row or column with already-placed digits, immediately ruling out most candidates.

More questions? See the full Thermo Sudoku guide.